At the moment I think it leans more towards frustration than fun, but if you could shift the balance the other way, this would be really terrific.
Things that are irritating:
- The vertical movement feels faster than the horizontal movement because the height of the characters that make up the snake/level is greater than the width.
- Switching screens is not great to begin with, as the player has to reorient quickly, but this is particularly bad when changing screens vertically, especially if you're rapidly heading for a wall on the new screen.
- The same thing goes for the green warp world. How could you better orient the player for where the snake is going to emerge?
- All of the warps could do with being 2-3 characters wide. It's not fun trying to hit a single square when the movement is often not precise enough to do this easily.
- I would strongly consider using any screen entry point as a de-facto checkpoint or implementing some kind of fast-travel. There's nothing more frustrating than trying to get back through a tricky section and then having to traverse multiple screens to find a letter or checkpoint that will take over from the letter you grabbed earlier, when every mistake sends you back to the letter at the end of a tricky section.
- Invisible walls in small doses are bearable. To have two worlds built around that mechanic is a bit much.
- If the game design is going to require me to die a lot then get me back into the game more quickly. The pop-up message adds an unnecessary delay and it also creates a sort of visual reset, especially if you just died in the area the message obscures.